Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to an imaging lens which is built in image pickup devices mounted in in-vehicle cameras, game consoles, security cameras, information terminals such as PCs, and home appliances with a camera function.
In the present invention, in terms of lens surface shape, a convex surface and a concave surface refer to a paraxial shape (shape near an optical axis). The angle of deviation here means the angle between the incident ray and the emerging ray passing through a lens.
Description of the Related Art
In recent years, there has been a general tendency that vehicles and home appliances have a camera function for increased safety and convenience. For example, in a vehicle with a camera for photographing the area behind the vehicle, the driver can check on a monitor what is going on behind the vehicle, so that the vehicle can be driven back safely. Furthermore, vehicles with a camera capable of photographing the area around the vehicle in all directions (forward, backward, left, and right) have also been introduced into the market. In such vehicles, the driver can see the entire surroundings of the vehicle on the monitor as if viewed from above the vehicle, so very high visibility and safety are assured. On the other hand, in home appliances, for example, a camera mounted in an air conditioner can offer high convenience when it is used to monitor the room condition and get information on the number of people in the room so as to adjust the direction of air flow and the flow rate for power saving. It is expected that the demand for products with camera functions as mentioned above will grow in the future and new products to meet the demand will be developed at an accelerated pace.
The imaging lens mounted in such products is required to be compact and provide a wide field of view to cope with the function of the product, and deliver high imaging performance throughout the image. In addition, since there is an increasing tendency toward low-priced products, the imaging lens is also strongly expected to be supplied at a low price.
Conventionally, the imaging lens described in JPA-2009-128654 (Patent Document 1) and the imaging lens described in JP-A-2010-276752 (Patent Document 2) are known as examples of imaging lenses with a wide field of view.
Patent Document 1 discloses a fish-eye lens system which includes, in order from an object side, a first group with negative refractive power, a second group with positive refractive power, and a third group with positive refractive power, in which the first group includes, in order from the object side, a meniscus first lens with negative refractive power having a convex surface on the object side, a meniscus second lens with negative refractive power having a convex surface on the object side, and a biconcave third lens having a highly refractive surface on an image side, and the second group includes a fourth lens with positive refractive power having a highly refractive surface on the object side, an aperture stop, and a cemented lens having a fifth lens with positive refractive power, a biconcave sixth lens, and a seventh lens with positive refractive power, and the third group includes an eighth lens having a highly refractive surface on the object side.
Patent Document 2 discloses a wide-angle lens which includes, in order from an object side, a negative meniscus first lens having a concave surface on an image side, a negative meniscus second lens having a concave surface on the image side, a third lens with positive refractive power, an aperture stop, and a fourth lens with positive refractive power, in which the second lens is a single-sided aspheric lens and the fourth lens is a double-sided aspheric lens.
The imaging lens described in Patent Document 1 is a fish-eye lens with high optical performance which uses eight constituent lenses and offers relatively high brightness with an F-value of 2.8 and a field of view of 210 degrees or more. However, since it uses a relatively large number of constituent lenses and glass lenses and includes a cemented lens, its manufacturing cost is high. In addition, its total track length is 20 mm or more, so it cannot be compact enough.
The imaging lens described in Patent Document 2 is a compact wide-angle lens with high optical performance which uses only four constituent lenses and offers a relatively high brightness with an F-value of 2.6 and a field of view of 190 degrees or more. It uses glass material for the first and third lenses and plastic material for the second and fourth lenses, so that it can be supplied at a relatively low price. However, the negative meniscus first lens, located nearest to the object, has a large diameter and to manufacture it using glass material involves a high degree of processing difficulty, which is an obstacle in an attempt to decrease the manufacturing cost. In the imaging lens, if plastic material is used for the first lens, field curvature and astigmatism which are too serious to be corrected by the third and fourth lenses would occur and it would be difficult to deliver high optical performance.